This invention relates generally to mass spectrometry, and in particular to laser desorption time-of-flight mass spectrometers.
Matrix-assisted laser desorption time-of-flight mass spectrometry is a recently developed technique which is particularly useful for the sensitive analysis of large biomolecules. Typically, a few microliters of solution containing sample molecules at concentrations of about 1 82 g/.mu.L are mixed with 10-20 .mu.L of a solution containing matrix molecules at concentrations of about 10 .mu.g/.mu.L. A few microliters of this mixture are then deposited on a suitable substrate and dried in air.
Once the sample has been introduced into the mass spectrometer, a pulsed laser is used to irradiate the sample on the substrate. The interaction of the laser radiation with the matrix molecules leads, by a process that is only partly understood today, to the formation and desorption of largely intact, ionized sample molecules. Predominantly these ions are of a type known as (M+H).sup.+ ions, that is, the neutral sample molecule (M) is ionized by the attachment of a proton. Negatively charged ions may also be formed.
Most frequently these ions are analyzed in so-called linear time-of-flight (TOF) mass spectrometers, that is the ions, once formed, are accelerated by an electric field and then allowed to travel in straight lines until they are detected. The transit time between ion formation and detection can be used to determine the mass of the species from which the ions are generated. A typical linear TOF system is described in U.S. Pat. No. 5,045,694 by Beavis and Chait.
Such linear devices provide only modest mass resolving power, e.g. 50-800, because they are unable to compensate for various known aberrations. A dominant aberration in such linear systems stems from the fact that the ions are formed with a wide distribution of initial velocities. This means that for an ion of a given mass there will be a distribution of arrival times at the detector that will limit the mass resolving power of such a device, since ions with more initial velocity in the forward direction will arrive sooner than ions with less initial velocity in the forward direction.
Techniques for compensating for such aberrations resulting from the initial velocity distribution in TOF mass spectrometers are well-known. The primary technique is to provide an electrostatic mirror, called a Reflectron, which reverses the direction of travel of the ions in such a way that the effects of these initial velocity distributions on ion transit times are eliminated. A recent review article describing such devices is "Time-of-flight Mass Spectrometry: An increasing Role in the Life Sciences", R. J. Cotter, Biomed. Env. Mass Spectrom., vol. 18,513-532 (1989).
Although the use of electrostatic mirrors for the analysis of ions formed by matrix-assisted laser desorption is well-known, the performance of such devices is less than optimal. This is primarily due to the fact that ions thus generated undergo significant rates of metastable decay thereby generating fragment ions during their passage through the mass spectrometer. These fragment ions are a source of error because they cannot easily be distinguished from parent ion peaks, or alternatively they can result in asymmetric peak broadening.
Thus the need exists for a laser desorption TOF mass spectrometer that can differentiate, and hence eliminate, the interference of fragment ions with parent ions of interest.